1. Field of the Invention
The present invention relates to an automatic focusing device for a camera such as, for example, a still camera, a video camera, etc., and more particularly to an automatic focusing device having an active range finding system utilizing triangulation and a passive range finding system utilizing image phase difference detection for operating the focusing mechanism according to thus obtained range data.
2. Description of the Prior Art
An example of a conventional automatic focus adjusting device is described in Japanese Patent Publication Laid-Open No. 212912/1988, which discloses a device having an active range finding system utilizing triangulation and a passive range finding system utilizing image phase difference detection for operating the focusing mechanism according to thus obtained range data. Initially, this system attempts to focus on an object using data from the passive range finding system. If the distance to the object cannot be obtained by the passive range finding system, the system uses its active range finding system to acquire range information. It then adjusts the lens focus to a predetermined extent according to the range data obtained from the active range finding system. Finally it adjusts the focus once again using range data from the passive range finding system.
The device first measures the distance to the object using passive range finding. If the result of the passive range measurement produces an unacceptably large degree of blur, which means focusing by the passive range finding system is difficult, then range data is taken from the active range finding system to move the focus to a point where focusing by the passive range finding system is possible. If the initial passive range measurement provides a small enough degree of blur, focusing can be completed using only passive range finding without requiring the use of the active range finding system.
In case of, for example, a 100 mm telephoto lens, the degree of blur detectable by the passive image phase difference detecting method is not satisfactory over the entire distance range from infinity to the closest focusing distance. With the above procedure, this inadequacy of the passive ranging system is overcome by resort to the active range finding.
Passive range finding is used unless it is found that the degree of blur is too large to permit satisfactory focus. When the degree of blur is very large, a tremendous amount of passive range data to make the decision to use the active range finding system. The large amount of passive range data requires a long time to calculate. As a consequence, ranging can take a substantial length of time.
The active range finding system, using triangulation, presents a problem of so-called background focusing due to focusing past the desired object. Background focusing is caused when light emitted to determine the distance to an object misses the object an strikes the background. This commonly happens when the object to be photographed consists of two persons standing side by side with the background showing between them, or in similar situations. Avoidance of background focusing demands careful attention from the photographer, thus increasing the workload of the photographer.
An active range finding system has a range measurement limit at the long-distance end of its range. This long-distance range limit results from the limited amount of emitted light available from suitable light emitting elements.
The active range finding system also has a problem at the short-distance end of its range due to parallax problems. Parallax problems are caused by positional relations between the range finding area and the object.
The above facts can also mean that it is possible for an active range finding system to be sufficiently effective at relatively close distance. The problem of background focusing due to focusing past the object can be overcome by increasing the range measuring distance by, for example, increasing the number of light emitting elements. An example of such method is disclosed in Japanese Patent Publication Laid-Open No. 223734/1987 owned by the applicant of the present invention.
Unlike active range finding systems, passive range finding systems are characteristically free from problems of parallax and are able to measure the range to the center of an object located anywhere from the closest focusing range to infinity. However, the area over which they are capable of measuring range is relatively small. An attempt to increase the measurable area presents new problems, such as an increase in the number of range finding image sensor elements.